1. Technical Field
The present invention relates to a counting device which counts the number of signals, and an interference type physical quantity sensor which measures the number of interference waveforms using the counting device and obtains physical quantities of an object to be measured.
2. Related Art
In the related art, there has been proposed a laser measurement device of a wavelength modulation type using a self-coupling effect of a semiconductor laser (see JP-A-2006-313080). FIG. 32 shows a configuration of such a laser measurement device. As shown in FIG. 32, the laser measurement device includes a semiconductor laser 201 which emits laser light to an object 210, a photodiode 202 which converts light output of the semiconductor laser 201 into an electrical signal, a lens 203 which condenses light from the semiconductor laser 201, which is then emitted to the object 210, and condenses return light from the object 210, which is then incident into the semiconductor laser 201, a laser driver 204 which drives the semiconductor laser 201 to alternatively repeat a first oscillation period during which an oscillation wavelength continuously increases and a second oscillation period during which the oscillation wave length continuously decreases, a transimpedance amplifier 205 which converts an output current of the photodiode 202 into a voltage and amplifies the voltage, a signal extraction circuit 206 which differentiates an output voltage of the transimpedance amplifier 205 twice, a counting device 207 which counts the number of mode hop pulses (MHPs) contained in an output voltage of the signal extraction circuit 206, a computing device 208 which calculates a distance to the object 210 and a speed of the object 210, and a display 209 which displays a result of the calculation by the computing device 208.
The laser driver 204 supplies a triangular wave driving current with repeated increase/decrease at a constant rate of change in terms of time, as an injection current, to the semiconductor laser 201. Thus, the semiconductor laser 201 is driven to alternate between the first oscillation period during which the oscillation wavelength continuously increases at a constant rate of change and the second oscillation period during which the oscillation wavelength continuously decreases at a constant rate of change. FIG. 33 shows a temporal change of the oscillation wavelength of the semiconductor laser 201. In FIG. 33, P1, P2, λa, λb, and Tt represent the first oscillation period, the second oscillation period, the minimum value of the oscillation wavelength for each oscillation period, the maximum value of the oscillation wavelength for each oscillation period, and a cycle of a triangular wave, respectively.
Laser light emitted from the semiconductor laser 201 is condensed by the lens 203 and then is incident into the object 210. Light reflected by the object 210 is condensed by the lens 203 and then is incident into the semiconductor laser 201. The photodiode 202 converts light output of the semiconductor laser 201 into a current. The transimpedance amplifier 205 converts an output voltage of the photodiode 202 into a voltage and amplifies the voltage, and the signal extraction circuit 206 differentiates an output signal of the transimpedance amplifier 205 twice. The counting device 207 counts the number of MHPs, which are contained in an output voltage of the signal extraction circuit 206, for each of the first oscillation period P1 and the second oscillation period P2. The computing device 208 calculates a distance to the object 210 and a speed of the object 210 based on the minimum oscillation wavelength λa and the maximum oscillation wavelength λb of the semiconductor laser 201, the number of MHPs for the first oscillation period P1, and the number of MHPs for the second oscillation period P2. When the number of MHPs is measured using the technology of such a self-coupling type laser measurement device, it is possible to calculate a vibration frequency of the object from the number of MHPs.
The above-mentioned laser measurement device has problems in that errors occur in physical quantities such as the calculated distance, the calculated vibration frequency due to an error of the number of MHPs counted by the counting device, which occurs when noise, such as disturbance light or the like, is counted as MHPs or uncountable MHPs are present due to omission of signals.
Therefore, the present inventor(s) has suggested a counting device which is capable of eliminating an effect of deficient counting or excessive counting by measuring a cycle of MHPs during a counting period, generating a frequency distribution of the cycle during the counting period from a result of the measurement, calculating a representative value of the cycle of MHPs from the frequency distribution, obtaining the total sum Ns of frequencies of a class which is equal to or less than a first predetermined multiple of the representative value, and the total sum Nw of frequencies in a class which is equal to or more than a second predetermined multiple of the representative value from the frequency distribution, and correcting a counting result of MHPs based on these frequencies Ns and Nw (see JP-A-2009-47676).
The counting device disclosed in JP-A-2009-47676 can achieve a generally good correction as long as a SNR (Signal to Noise Ratio) is not extremely lowered.
However, with the counting device disclosed in JP-A-2009-47676, if a signal strength is extremely high in short range measurement as compared to a hysteresis width, there may be a case where chattering occurs near a binarization threshold value in a signal input to the counting device due to noise having a frequency higher than that of MHP and signals having a short cycle or signals having a cycle which is about half the original cycle of MHP are frequently generated. In this case, since the cycle shorter than the original cycle of MHP becomes a representative value of a distribution of cycle, there arises a problem in that a counting result of MHP can not be properly corrected and becomes larger by several times or so than its original value.